Cambios de frecuencia e intensidad de El Niño y La Niña asociados a un reciente cambio de régimen frente al Peru costero Changes Frequency and Intensity of El Niño and La Niña associated to a recent regime shifts off coastal Peru

Cambios de frecuencia e intensidad de El Niño y La Niña asociados a un reciente cambio de régimen frente al Peru costero Changes Frequency and Intensity of El Niño and La Niña associated to a recent regime shifts off coastal Peru Sara Purca1 and Tarsicio Antezana2 and Roberto Riquelme3 1

Centro de Investigaciones en Modelado Oceanográfico y Biológico Pesquero (CIMOBP), Instituto del Mar del Peru, Apdo. 22, Lima, Peru. 2 Departamento de Oceanografía. Universidad de Concepción. Chile. 3 Departamento de Ingeniería Matemática. Universidad de Concepción. Chile. Resumen Sobrepuesto a eventos El Niño/La Niña (ENLN), los cambios de régimen han sido detectados a través de análisis de series de tiempo de TSM para toda la cuenca del Pacífico Sur y el Océano Pacífico Tropical Este (PTE). La serie de tiempo de temperatura superficial del mar (TSM) de la costa peruana y el indice de El Niño y la Oscilación del Sur (ENOS) fueron analizados de 1950 a 2002. El análisis de datos peruanos de TSM in situ en 5 lugares regularmente dispersos permite detectar un incremento de eventos ENLN con menor frecuencia que el homologo cambio de regimen de PTE. Los datos de TSM peruanos fueron representados por el Indice de Oscilación Peruano (IOP), y el coeficiente de correlación significativo fue observado entre el IOP y los indices de la región El Niño y PDO. Así, El Niño, el periodo cálido del ciclo ENOS mostró un retraso negativo de 2-3 meses sobre el IOP con respecto a los indices oceánicos para la fase fría del cambio de régimen (1950-1969), y un retraso positivo de 3-6 meses para la fase cálida (1980-1999). La correlación espacial para las bandas de frecuencia de ENLN (baja, ENOS y alta frecuencia) entre IOP y TSM global mostraron una banda de frecuencia ENOS (68 años) dominante a lo largo de la costa peruana, entonces esa dominancia se extendió en la banda de alta frecuencia (0-2 años), finalmente la señal ENOS se extendió en la banda de baja frecuencia (7-14 años). El efecto de los procesos planetarios de baja frecuencia en las regiones costeras ha sido observado ampliamente en la señal ENOS en la región costera y sugiere que el IOP sostiene la señal ENOS frente al Perú. Abstract Overimpossed to El Niño/La Niña (ENLN) events, regime shifts have been detected through SST time series analysis for entire South Pacific basin and Eastern Tropical Pacific Ocean (ETP). Sea surface temperature (SST) time series of Peruvian coast and El Niño-Southern Oscillation (ENSO) index were analyzed at 1950 to 2002. The analysis in situ SST Peruvian data at 5 locations evenly dispersed allows to detect an increase ENLN event with lower frequency than the homologous ETP regime shift. The Peruvian SST data was represented by the Peruvian Index Oscillation (POI), and the significantly coefficient correlation was observed between the POI and the indexes of EL NIÑO region and PDO. Therefore, El Niño, the warm period of the ENSO cycle showed 2-3 months negative lag on the POI with respect to the oceanic indexes for the cold phase of the regime shift (1950-1969), and a 3-6 months positive lag for the warm phase (1980-1999). The spatial correlation of ENLN bands frequencies (low, ENSO and high frequencies) between POI and SST global showed an ENSO frequency band (6-8 y) dominant along the Peruvian coast, therefore that dominance was extended in high frequency band (0-2 y), finally the ENSO signal was extended in low frequency band (7-14 y).

The effect of low frequency planetary processes in coastal regions have been observed ampleness the ENSO signal in coastal region and suggests the POI will be holding the ENSO signal off Peru. Introduction The ENSO affects oceanography and atmospheric conditions along the Peruvian coast dramatically [Philander 1990]. The coupling between regime shift signal of the SST Pacific basin and SST Peruvian coast has been suggested by correlation analysis [Trenberth 1999, Montecinos et al. 2003]. The changes El Niño-La Niña events have been described by climate changes and regimen shift in SST [Fedorov and Philander 2000, An and Wang 2000, Mitchell and Wallace 1996]. Recent studies decadal and interdecadal oscillation associated with regimen shift, numerous papers broadly review in northern [Nakamura et al 1997] and Southern Hemispheres [Linsley et al 2000 and Tourre et al 2001] and associated them with climate changes [Timmenman et al 1999, Philander and Fedorov 2000, Wang and An 2001]. The first evidence the change frequency and intensity for El Niño and La Niña events have been evolution with the multiple scales in the Peruvian coast, Tropics and South Pacific Ocean, and its was studied for Ramusson and Wallace 1983. Therefore, the main aim of this work is to determine the variability of the warm and cold events along the Peruvian coast and to establish how it associated with the regime shift SST of the South Tropical Pacific after El Niño 19821983. Materials and methods Data and processing Monthly average SST during the 1950-1999 period was compiled for 5 evenly spaced coastal stations along the coast of Peru and analyzed together with oceanic Index (NIÑO1+2, NIÑO3, NIÑO4, TNI and SOI) with period based 1950-1999. The coastal stations were following: Talara (04°34’24’’S) (81°17’18’’W), Chicama (07°42’00’’S) (79°27’00’’W), Chimbote (09°04’00’’S) (78°36’00’’W), Callao (12°13’00’’S) (77°09’00’’W), San Juan de Marcona (15°21’00’’S) (75°09’00’’W). Statistical time-series analysis followed closely Zhang et al 1997. Annual cycles and El Niño-La Niña period were explored as described by Deser and Wallace [1990]. In order to capture El Niño and La Niña mode Peruvian stations were utilized the first principal component from anomalies SST coastal stations, which we called Peruvian Oscillation Index (POI); Index Niño 1+2 (0-10°S)(90°-80°W), Niño 3 (5°N-5°S)(150°W-90°W), Niño 4 (5°N-5°S) (160°E-150°W), Niño 3+4 (5°N-5°S)(170-120°W), Trans Niño Index (Niño 1+2)-(Niño 4) TNI and SOI were utilized in order to compare our data with Eastern Pacific Ocean and South Pacific Ocean. Intensity of frequencies of the SST data was examined by spectral analysis techniques described by Torrence and Compo in 1998, for based period 1950-1999. For the statistical analysis all the origin al series were transformed into standardized anomalies to extract the tendency and the stacionariety to him, with a period bases 1950 to 1969 and 1980 to 1999. The period is based corresponding to each regime 1950-1969 and 1980- 1999 for the cross correlation. Excluding the decade from 1970, because when each coastal station is analyzed separately the change of slight regime presents desfaces. On the other hand the anomalies filtered with a model AR (3) and by means of the distribution of the residuos give a limit them of confidence of the spectral analyses and wavelet. Established the parameters of entrance of the series it is determined you limit them of the frequency of the series by means of windows of Fourier. Finally it is observed the drastic changes in different periods (multiscale) throughout the global series. The variance of the spectral for the wavelet determines by means of resamples of

the series and average of the power of the wavelet of each resamples (1000 times), this way was obtained you limit them of confidence for the location of the drastic changes (Torrence and Compo 1998). Results The standardized SST anomalies of the five coastal locations represented for Peruvian Index Oscillation (POI), NIÑO3 and TNI exhibited El Niño 1982-1983 and 1997-1998, the POI had a strongest value during El Niño period (Figure 1).

Figure 1. First principal component anomalies of SST along the Peruvian coast called Peruvian index Oscillation (POI) for based period 1950-1999. The positive anomalies showed El Niño events and negative anomalies showed La Niña events. The TNI and The ENOS index were period based 1950 to 1999. The regime shift centered in 1975-1976 [Nitta and Yamada 1989 and Montecinos et al. 2003], encompassing a succession of numerous warm and cold periods, we refer to El Niño and La Niña events to those outstanding warm or cold periods of positive or negative standardized anomalies without pre-established thresholds, as determination of their deeper limit intensities was not attempted in this work, we observed of their upper limit intensities. During the cold phase (1950-1975), six El Niño events and seven La Niña events were identified; during the warm phase (1975-1999) five El Niño events and six La Niña events were identified. El Niño events had similar intensities during the cold phase of regime shift. El Niño events occurred in a quasi periodic mode during the last decade of the cold phase. Instead, during the warm phase El Niño events occurred randomly. The interannual signal remained clear and greatly uncharged by the interdecadal signal. When comparing the distribution of SST from the Peruvian coast and the distribution of pressure anomalies of the South Pacific differences in the frequency of the interdecadal signal appeared. Cross correlation of SST (15 months) for five coastal station during 1950-1969 and 1980 to 1999 periods (Figure 2 a and b) showed no lag during cold phase and a warm phase. Cross correlation of SST (15 months) between coastal station and oceanic indexes for the 1950-1969 and 1980 to 1999 phases (Figure 2 c and d) showed a negative time lag of 5 month for the cold phase and a positive lag of 6 months for the warm phase.

Figure 2. Cross correlation of SST (15 months) for five coastal station during 1950-1969 and 1980 to 1999 periods (Figure 3 a and b) showed no lag during cold phase and a warm phase. Cross correlation of SST (15 months) between coastal station and oceanic indexes for the 1950-1969 and 1980 to 1999 phases (Figure 3 c and d) showed a negative time lag of 5 month for the cold phase and a positive lag of 6 months for the warm phase.

Figure 3. The spatial correlation map frequency represents the 0-3 yr between POI and SST global. The period based 1950 to 1969.

Figure 4. The spatial correlation map frequency represents the 3-6 yr between POI and SST global. The period based 1950 to 1969.

Figure 5. The spatial correlation map frequency represents the 7-10 yr between POI and SST global. The period based 1950 to 1969.

Time- frequency charts between SST of the coastal station during 1950-1999 had 3 and 6 years highlight power variance on the global spectrum (Figure 3). During the cold phase, a positive spatial correlation was presented between POI and SST global (1950-1969) by 0 and 3 years frequency rand in the Eastern Equatorial Pacific area (Figure 4), while 3 and 6 frequencies rand, a negative spatial correlation was observed in the Eastern Tropical Pacific (Figure 5). Finally, a 7 and 10 frequency rand, a positive spatial correlation was observed along the Equatorial Pacific frequencies. Discussion Several indexes have attempted to describe low frequency changes in the Equatorial Pacific mainly those associated with ENOS, each representing a phase of its longitudinal evolution [Larkin and Harrison 2001]. SOI and El NIÑO3 region represents El Niño-La Niña cycle for the entire Equatorial Pacific basin, index for El Niño 3+4 represents time and space evolution of ENOS in the 120º-150ºW; Index for El Niño 1+2 in the 80º-100ºW latitudinal range, closely represents by extension the south-American coastal conditions, however, local conditions are not necessarily described by these indexes as coastal processes likely affect the evolution of El Niño-La Niña [Deser and Wallace 1987; Trenberth 1996; Philander 1990; Trenberth and Stepaniak 2001]. Local data although scarce and hard to come by, allow proposing a more appropriate index. SST data for the Callao, Chicama and Talara have been corrected and partially used in earlier studies associated to El Niño and La Niña in the Oceanic Pacific [Rassmusson and Wallace 1983; Deser and Wallace 1987; Mitchell and Wallace 1996] or along the coast of South America [Enfield and Allen 1980]. The POI showed a similar regime-shift centered around 1975 to that shift described for the SOI data [Nitta and Yamada, 1989, Montecinos et al. 2003], but the POI showed longer period (< 28yr). Differences may be attributed to the effect of more intense effects of El Niño 1982-83 and El Niño 1997-1998 in the coastal region than in the oceanic region (Figure 1). The lower frequency in the interdecadal frequency of the POI is also shown when it is compared to indexes for regions El Niño 3+4 and El Niño 1+2. Changes in the regime-shift in both coastal and oceanic regions NiÑO 1+2, NIÑO 3, NIÑO 3+4 has an immediate consequence in the occurrence of El Niño. During the cold phase (1950-1969), the negative lag of 5 month in the occurrence of El Niño in the Peruvian coast as compared to the SOI and El NIÑO 3+4 index for these regions is consistent to previous finding [Rasmusson and Carpenter 1982]. These authors attributed such a lag to the Kelvin wave displacement velocity. For the warm phase instead, the 6 month positive lag for the Peruvian coast with respect to the El NIÑO 3+4 region, could be associated to resonance effects of a longer than El Niño frequency in the coastal region. Coastal-trapped waves have been associated to El Niño events along the coast of South America [Shaffer et al 1997]. A latitudinal evolution of El Niño event was established [Enfield and Allen, 1980] according to SST and sea level data for Alaska to Valparaiso coastal stations, which include only Chicama and Callao in Peru. We suggest that these results contribute to better understanding of low frequency events in the coastal region and to improve models for ENSO-like on Eastern Tropical Pacific. Acknowledgments Sara Purca thanks for support from MECESUP/UCO-0002 Scholarship, Scholarship cod. NºA/98/01108 by Deutscher Akademischer Austausch Dienst (DAAD), and Scholarship from Escuela de Graduados-Universidad de Concepción. Roberto Riquelme is grateful for support from Proyecto Interno de Investigación. UdeC Nº 201.013.019-1.0.

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